In an exhaust gas recirculation device for decreasing the emission of NO.sub.x components in the exhaust gas of an internal combustion engine, it is necessary to maintain an appropriate EGR ratio adapted to an engine condition in order to obtain a maximum EGR effect while maintaining a maximum fuel consumption efficiency, and in order to prevent an adverse effect from occurring during the operation of the engine. In conventional EGR devices which are widely used, various vacuum control mechanisms are utilized for effecting the control of the EGR ratio.
In recent developments of the EGR device, a device controlled by a computer has been proposed for controlling the EGR ratio. In such a system, values of the parameters of the EGR ratio, for example, values of the intake pressure, opening of the EGR valve, etc., are preliminarily stored in a memory of the computer with respect to values indicating the conditions of the engine intake system, for example, values indicating the engine rotational speed and the amount of air intake. When the engine is operating, a value of the parameter of the EGR ratio is read out from the memory by sensing the engine rotational speed and the amount of intake air. The read-out value is utilized for feed-back or feed-forward controlling of the amount of exhaust gas so that a desired value of the EGR ratio adapted to a corresponding engine condition is obtained.
Generally speaking, the temperature conditions of the engine, for example, the temperature of the intake air, the cooling water, the engine cylinder block cooling water, are very important for determining the EGR ratio. For example, the EGR operation should be stopped or decreased when the engine is being warmed or when it is overheated. Therefore, various mechanisms have been proposed for utilization in the conventional vacuum-operated EGR system for controlling the transmission of a vacuum signal from the engine intake pipe to the EGR valve when the engine is warmed from a cold engine or when it is overheated.
Such temperature control of the EGR ratio is also required in the above-mentioned computer-controlled type of EGR device.